1. Field of the Invention
The present invention relates to a method of producing a micro structure suitable for the production of a liquid jet recording head (which may be referred to as a liquid discharge head) for generating small droplets of a recording solution used in an ink jet recording method, a method of producing a liquid jet recording head using this method and a liquid jet recording head obtained thereby. Particularly, the present invention relates to a technique useful for a method for producing a liquid flow path shape for achieving a high speed recording process and a head thereof.
Moreover, the present invention relates to an ink jet head having an improvement of ink discharge characteristics according to a method of producing an ink jet head.
2. Related Background Art
A liquid discharge head, which is adapted to an ink jet recording method (liquid discharge recording method) for performing recording by discharging a recording liquid such as ink, generally includes a liquid flow path, a liquid discharge energy generating unit formed on parts of the liquid flow path and a fine recording liquid discharge port (hereinafter, referred to as “orifice”) for discharging the liquid of the liquid flow path by a heat energy of the liquid discharge energy generating unit. Conventionally, methods for producing such a liquid discharge recording head include, for example, (1) a method in which a through hole for supplying ink is formed on an element substrate with heaters for generating a heat energy to discharge a liquid, a drive circuit for driving these heaters and so on, the wall of the liquid flow path is patterned by negative photosensitive resist and a plate with an ink discharge port is bonded thereto by electroforming or excimer laser working; and (2) a method in which an element substrate formed in the same way as the above method is prepared, a liquid flow path and an ink discharge port are formed on a resin film (preferably, polyimide) coated with an adhesive layer by excimer laser and then a processed plate of a liquid flow path structure and the element substrate are bonded by applying a heat and a pressure.
In the ink jet head produced by the above methods, the distance between the heaters and the discharge port affecting a discharge amount must be as short as possible in order to enable the discharge of a small droplet of liquid for high quality recording. For this, there is a need to decrease the height of the liquid flow path and decrease the size of a discharge chamber or discharge port acting as a bubble generating chamber which is a part of the liquid flow path and is contacted with the liquid discharge energy generating unit. That is, in order to enable the discharge of a small droplet of liquid by the head produced by the above methods, the liquid flow path structure to be laminated on the substrate needs to be thinned. However, it is very difficult to process the thin liquid flow path structure plate at a high precision and bond it to the substrate.
To solve the problems of these methods, Japanese Patent Application Laid-Open No. 6-45242 discloses an ink jet head production method (hereinafter, abbreviated as “patterning method”) in which the pattern of a liquid flow path is formed of a photosensitive material on a substrate with a liquid discharge energy generating element, a coating resin layer is coated on the substrate for coating the pattern, an ink discharge port communicating with the pattern of the liquid flow path is formed on the coating resin layer and then the photosensitive material used in the pattern is removed. In this head production method, the photosensitive material is a positive resist from a viewpoint of removal convenience. In addition, according to this method, since a photolithography technique of a semiconductor is applied, it is possible to perform a fine working at a very high precision for the formation of the liquid flow path, discharge port and so on. However, in the method which has adapted to this semiconductor production method, basically, a shape change of the regions near the liquid flow path and the discharge port is limited to the change in a two-dimensional direction in parallel with the element substrate. In other words, since the patterns of the liquid flow path and the discharge port are made of the photosensitive material, the photosensitive material layer cannot be partially multilayered. Thus, it is impossible to obtain a desired pattern with alteration in the height direction to the pattern of the liquid flow path and the like (that is, a shape in a height direction from the element substrate is substantially same and limited). As a result, this becomes an obstacle in designing the liquid flow path to implement a stable discharging at a high speed.
Japanese Patent Application Laid-Open No. 10-291317 discloses an implementation of a change of the shape of a liquid flow path in a three-dimensional direction, that is, in an interplanar direction in parallel with the element substrate and in a height direction from an element substrate, by controlling a processing depth of a resin film by partially changing the opacity of a laser mask in the excimer laser working of the liquid flow path. Although such a control in the depth direction in the laser working is possible in principle, the excimer laser used in this working is different from the one used in exposure of a semiconductor but is a laser of a high brightness used in a broad band and suppresses a deviation of illuminance within a laser irradiation surface, thereby making it difficult to implement the stabilization of a laser illuminance. Particularly, in an ink jet head of high image quality, the unevenness of the discharge characteristics caused by a deviation of a processed shape between discharge nozzles is recognized as a spot in an image, thus it is a big task to achieve the improvement of a processing precision.
Moreover, there are frequent occasions when a fine patterning is impossible due to a taper on a laser working surface.
By the way, Japanese Patent Application Laid-Open No. 4-216952 discloses a method in which a first layer of negative resist is formed on a substrate, a latent image of a desired pattern is formed, the first layer is coated by a second layer of negative resist, a latent image of a desired pattern is formed only no the second layer, and finally the latent image of the pattern of the upper and lower layers are developed. In this method, the negative resists of the two upper and lower layers have a different sensitive wavelength region from each other. The both upper and lower resist layers are ones sensitive to ultra violet (UV) rays, or the negative upper resist layer is one sensitive to ultra violet rays and the negative lower resist layer is one sensitive to ionizing radiation such as deep-UV, electron beams, X-rays and so on. According to this method, by using the negative resists of the two upper and lower layers each having a different sensitive wavelength region, a pattern latent image changed in shape can be formed in a height direction from the substrate as well as in the direction parallel to the substrate.
Hence, the present inventors studied the application of the technique disclosed in Japanese Patent Application Laid-Open No. 4-216952 to the above-mentioned pattern forming method and thought that, if the technique of Japanese Patent Application Laid-Open No. 4-216952 is applied to the formation of the pattern of the liquid flow path in the pattern forming method, the height of a positive resist which forms the pattern of the liquid flow path can be changed locally.
Practically, it has been attempted to use an alkali development positive photoresist comprising a composite of alkali soluble resin (Novolak resin or polyvinylphenol) and naphthoquinone diazide derivative as the resist which is soluble, removable and sensitive to UV; polymethylisopropenyl ketone (PMIPK) as the resist sensitive to ionizing radiation; further, to form upper and lower layers having a different pattern with respect to the substrate, as disclosed in Japanese Patent Application Laid-Open No. 4-216952. However, this alkali development positive photoresist could not be adapted to the pattern formation of two layers since it is instantly dissolved in a developing solution of PMIPK.
For this reason, this invention is focused on finding a combination of positive photosensitive materials of upper and lower layers capable of forming a pattern that is changed in shape in a height direction with respect to the substrate in the pattern forming method.